... i believe the drains in the floors and walls were plumbed in cast iron...

i'm fairly certain the thing is vented properly...

Click to expand...

venting has nothing to do with the visibility of the standing trap water between showers. Unless the trap loses all or almost all of its water... which is not the issue here.

Can you check and double check what the pipes are made of? First things first.

If it gurgles LOUD, then there is something happening in the transfer of air and water in the pipe while draining, and that has a lot to do with venting and the slope of the drain. Is his drain pipe pitched too steeply? Is the venting really up to the requirements?

Something may be causing the drain to suck or burp a little air while the shower is running. Then, that noise of water falling down a pipe, being amplified in a tube, coming back out at you, every half second, sounds disgusting. But this is not a diagnosis. Not yet.

My shower makes noise in the morning too.
It got louder after I snaked it.

I have ABS pipes, so the water running down the pipe will make more noise than if it were cast iron.

Guess I'm going to have to start singing in the shower to cover up the water noise.
That, and there is this water coming out of the shower head making noise too.
Now I gotta sing a little louder.
Geez, I hope I'm not waking up the neigbors now.

Put a 5 gallon bucket of water into a toilet...the water gurgles as it finishes flushing because it's a full S-trap.
I'm with Genius on this, I'll wager you either have a steep pitch on the drainage or an S-trap.
As for Terry's answer...please have mercy on the neighbors, leave it to Jamie.

If they don't mind possibly showering in a pond of water, maybe one of the floor drain "waterless" adapters will work. It is an insert with a plastic elephant trunk which closes and coils slightly when no water is flowing, but straightens out and opens up for water flow. It would direct the water closer to the trap so it might make less noise.